The present invention relates to apparatus for separating sheets from a stack so that the sheets may be fed to sheet processing apparatus such as printers. More particularly, the invention relates to sheet separating apparatus which operates on the "roll-wave" principle to separate sheets from a stack of sheets. This roll-wave principle of sheet separation was first described in U.S. Pat. No. 3,008,709 to W. S. Buslik as well as in an extensive number of subsequent publications including the publication, "Paper Feed Wheel," R. E. Hunt, IBM Technical Disclosure Bulletin, January 1978, page 2933; "Trailing Edge Paper Feeding Apparatus", R. E. Hunt, IBM Technical Disclosure Bulletin, October 1977, page 1678; "Combing Wheel Paper Feed," D. F. Colglazier, E. P. Kollar and F. R. Mares, IBM Technical Disclosure Bulletin, November 1977, page 2117; "Improved Shingler Design," E. J. Friery and B. H. Kunz, IBM Technical Disclosure Bulletin, December 1977, page 2579; "Sheet Shingler," J. L. Fallon, R. E. Hunt, E. P. Kollar, and J. H. Rhodes, Jr., IBM Technical Disclosure Bulletin, July 1978, page 477. As set forth in the basic patent and subsequent publications, the roll-wave principle involves the recognition that by urging a circular member such as a ball or roller into engagement with the surface of the uppermost sheet in a stack of paper or similar sheet material while applying only a minimum of pressure which would be necessary to form a slight deformity or depression in the several uppermost sheets beneath the circular element, then when the circular element is moved across the surface, and particularly if the circular element is free rolling, the slight indentation in the uppermost sheets will shift or roll like a wave following the moving circular element. This wave will in effect be transmitted to the edge of the stack of sheets and will result in the shingling of the uppermost sheets which were subject to the roll-wave depression.
Once the sheets are thus shingled, they are much easier to separate from the top of the stack with conventional sheet feed apparatus as described in the above art. During the period in the cycle when the shingled top sheet or sheets are being removed from the stack by the sheet feed apparatus, it is desirable that the circular member be withdrawn from engagement with the paper so that it will in no way impede the removal operations. In the prior art this is accomplished by a withdrawal expedient which lifts the circular shingling member away from the paper stack.
Among the most significant aspects of roll-wave paper shingling is the maintenance of a preselected constant pressure by the circular or roller element upon the surface of the uppermost sheet in the stack. If the pressure is too slight, there will be substantially no indentation and consequently no roll-wave to create the shingling. On the other hand, if the pressure is too great, paper buckling and damage may take place.